[Joakal]

An official letterhead letter be a cheap way to demand reimbursement. The letter can say "<person> wishes to cease employment and requests reimbursement for $<amount> for <work> by <date>. Please avoid delays otherwise there may be further costs." The employer can then settle to avoid further costs for both sides. I don't believe that such demands mean you are required to take them to court. So, it's like a legal bluff.

That said, the work you gave them belongs to you unless you stated otherwise (look through your logs). If the employer uses it, then you can send them an invoice for a license to use your IP with a notice that there'll be annual charges starting <date>. The employer will want to come to you for a full IP release. Otherwise, send an overdue notice then later send the unpaid invoice to collections after the overdue date if the employer refuses to pay.

Or release the code/work to GitHub or similar. You own the code (unless waived otherwise).

Note: The employer sounds quite charming to you, you'll need to plan out what to do before meetings including attempts to 'understand your plight' for lower but legally binding settlement (eg undesirable $100 reimbursement). But most important: stick to your objectives, not theirs. Maybe bring someone who can provide strong support which you can say they are present 'as a witness'. Maybe take a look in seduction ;)

IANAL, you should seek your own answers from someone more local at least. There's many ways to go about it like I listed above.